The present invention relates to a method of manufacturing a planar type lens which is suitably used for a screen for a back projection type projector, for example.
Recently, a back projection type projector using a liquid crystal light valve or a CRT has been actively developed as a large-screen display for HDTV (Hi-vision), a theater or the like.
FIG. 1 schematically shows the construction of a conventional back projection type projector.
A box type projector is illustrated as an example. Projection picture light L from a picture projection unit 101, for example, is reflected by a reflection mirror 102 and guided to a translucent type screen 105. The translucent type screen 105 comprises a Fresnel lens 103, and a lenticular lens 104 which normally extends in the vertical direction. The projection picture light L which is incident from the back surface of the translucent type screen 105 is set to substantially parallel light by the Fresnel lens 103, and then diffused mainly in the horizontal direction by the lenticular lens 104.
As shown in FIGS. 2A and 2B, the lenticular lens 104 is provided with projecting portions 104a extending in the vertical direction at the back side (light emission side), and black stripes 104b which absorb external light to enhance the screen contrast are provided to the projecting portions 104a. For example, after acrylic resin is subjected to extrusion molding to have the shape of the lenticular lens 104 containing the projecting portions 104a, and then only the projecting portions 104a are subjected to black print to form the back stripes 104b.
As shown in FIG. 2B, the width w of the black stripes 104b is normally set to be 0.3 to 0.4 time of the pitch p of the lenticular lens 104.
However, in the translucent type screen using the lenticular lens as described above, a wide angle of visibility can be obtained in the horizontal direction because the light is widely diffused in the horizontal direction. However, it has a disadvantage that the angle of visibility is small in the vertical direction because the light is diffused in only the narrow range in the vertical direction. In order to overcome this disadvantage, a structure having a combination of a lenticular lens extending in the vertical direction and a lenticular lens extending in the horizontal direction is known, however, it has a problem that the part cost and the manufacturing cost rise up because of the number of parts is increased. Further, there is a problem that the thickness of the screen is increased and the weight of the screen is increased because the lamination number of the screen is increased, and also the effect of the multiple reflection between respective layers is intensified.
Further, as described above, when the black stripes are provided to enhance the contrast, it is necessary that the projecting portions for black print are provided at the light emission side of the lenticular lens, and in addition it is necessary that the projecting portions are designed to have such a width that they do not obstruct the emission light. Therefore, the area rate of the external light absorption portion based on the black stripes is normally limited to about 30 to 40%. Therefore, the effect of enhancing the contrast is relatively low.
Therefore, in place of the lenticular lens, much attention has been paid to a translucent screen based on a planar type lens which is constructed by two-dimensionally arranging transparent fine spheres (see U.S. Pat. No. 2,378,252, U.S. Pat. No. 3,552,822, Japanese Utility Model Registration No. 2513508, for example), and studies and developments thereof have been performed to practically use the translucent screen for a large-screen high-definition display.
The construction which was previously proposed by the applicant of this application in Japanese Unexamined Patent Application No. Hei-9-100590 (filed, Apr. 17, 1997) will be described with reference to FIGS. 3 to 5, for example.
FIG. 3 shows the main construction of a back projection type projector of open type. Projection picture light L from a picture projection unit 21 is diffused forwardly through a translucent type screen 10 comprising a Fresnel lens 22 and a planar type lens 23. The planar type lens 23 is constructed by two-dimensionally arranging transparent fine spheres 2 such as glass beads in a closest packed structure. Accordingly, the projection picture light L can be diffused in a wide range in each of the horizontal and vertical directions by one layer comprising the transparent fine spheres 2.
FIG. 4 shows a back projection type projector of box type, and projection picture light L from a picture projection portion 21 disposed in a housing 25 is reflected by a reflection mirror 24, and diffused forwardly through a translucent screen 10 comprising a Fresnel lens 22 and a planar type lens 23 comprising transparent fine spheres 2.
FIG. 5 shows a planar type lens having the most basic construction in ones described in the above application.
In the planar type lens 23 having the most basic construction, the many transparent fine spheres 2 such as glass beads adhere onto a transparent substrate 1 such as a glass plate or the like through a colored layer (light absorption layer) having a sticky or adhesive function. Each transparent fine sphere 2 is buried in the colored layer 3 so as to be exposed from the colored layer 3 at the light incident side by about 50% of its diameter, and brought into contact with the transparent substrate 1 at the light emission side thereof.
The incident light L.sub.in which is incident through the Fresnel lens (not shown) is converged by each transparent fine sphere 2 as shown in the figure, transmitted in the neighborhood of the contact portion between each transparent fine sphere 2 and the transparent substrate 1, diffused and emitted. L.sub.out represents emitted light. On the other hand, most of external light L.sub.ex which is incident from the transparent substrate 1 side is absorbed by the colored layer 3, and thus reduction in contrast due to reflection of the external light L.sub.ex is suppressed.
At this time, in the planar type lens 23, the area rate of the light absorption layer at the light emission side by the colored layer 3 can be set to about 80% or more, for example. Accordingly, the reduction in contrast due to the reflection of the external light L.sub.ex can be greatly suppressed, and thus there can be implemented a screen which is hardly affected by the external light and has high contrast.
In the above application, the planar type lens 23 is manufactured as follows.
That is, first, the colored layer 3 serving as a sticky or adhesive layer is formed on the transparent substrate 1, and many transparent fine spheres 2 are scattered onto the colored layer 3. Thereafter, the transparent fine spheres 2 are pressed from the upper side thereof so as to be pushed into the colored layer 3.
According to such a method, however, when the transparent fine spheres 2 are pressed from the upper side thereof, the transparent fine spheres 2 are rotated, and thus the colored layer 3 adheres to the surface of the exposed portion, thereby inducing reduction in transmittance, thus reduction in brightness of the screen is induced.
Further, there are some portions where the colored layer 3 remains at a thickness of about several .mu.m between the transparent fine spheres 2 and the transparent substrate 1, so that the reduction of the transmittance, and thus the reduction in brightness of the screen is induced.
Further, it is normally needed to increase the temperature in order to reduce the viscosity of the colored layer 3 when the transparent fine spheres 2 are pushed in, and thus relatively large-scale facilities are needed to increase the temperature and cool. Further, occurrence of warpage of the transparent substrate 1 due to heat is a problem which cannot be neglected particularly for the large-screen display.